A vertical cavity surface emitting LASER (VCSEL) is a semiconductor laser, which emits light in a direction perpendicular to a substrate surface. The VCSEL has a feature, compared with an end-face emitting type semiconductor laser, of low cost, of low power consumption, small size, high performance, and being easily integrated two-dimensionally.
The vertical cavity surface emitting laser has a resonator structure that has a resonator region including an active layer, and upper and lower reflection mirrors provided above and below the resonator region, respectively (See Patent Document 1). Accordingly, the resonator region has a predetermined optical thickness so that light with wavelength of λ in the resonator region in order to obtain light with an oscillation wavelength of λ. The upper and lower reflection mirrors are DBRs (Distributed Bragg Reflector) formed by laminating materials having different refraction indices, i.e. a low refraction index material and a high refraction index material, alternately. In the DBR, the low and high refraction index materials are formed so that optical thicknesses normalized by the refraction indices of the respective materials are λ/4, in order to obtain high reflectance where the wavelength is λ.
Moreover, the vertical cavity surface emitting laser is often provided with an electric current narrowing region in the Bragg reflector. There is an effect of lowering a threshold current, since a transparent electric current density of the active layer can be achieved with a low electric current according to the electric current narrowing. Furthermore, by giving a refraction index difference in a transverse direction, it is also effective for transverse mode control.
Among structures of the vertical cavity surface emitting laser, there is an “intracavity structure”, in which a contact layer contacting an electrode is provided in the middle of the upper Bragg reflector and electrodes are arranged so as to surround a region where a mode control exists (See non-patent document 1). The intracavity structure may be employed according to a reason such as decreasing an influence from an increase in temperature of a layer above the active layer in the case of performing a process in the middle of the upper Bragg reflector.
Patent document 2 discloses an atomic oscillator provided with an optical system in which an end-face emitting type laser diode (coherent light source) is provided on a base, a passive optical element, a gas cell, and a photodiode (waveguide type light receiving element: light detector) provided on the base, which are serially arranged along a surface direction on a substrate. The respective elements are electrically connected to the substrate.
Non-patent document 2 discloses fabrication techniques usually applied to microelectromechanical systems (MEMS) used to reduce the size and operating power of the core physical assembly of an atomic clock. With a volume of 9.5 mm3, a fractional frequency instability of 2.5×10−10 at 1 s of integration, and dissipating less than 75 mW of power, the device has the potential to bring atomically precise timing to hand-held, battery-operated devices. In addition, the design and fabrication process allows for wafer-level assembly of the structures, enabling low-cost mass-production of thousands of identical units with the same process sequence, and easy integration with other electronics.
Non-patent document 3 discloses a combination of microelectromechanical systems (MEMS) fabrication with atomic clocks, and gives an overview of microfabrication techniques used for chip-scale atomic clocks (CSACs), including the fabrication and integration of the critical components. Furthermore, the performance of MEMS clocks is evaluated in terms of frequency stability and sensitivity to external parameters, size, and power consumption.
Non-patent document 4 discloses a spectroscopic technique of coherent population trapping (CPT) which enables an all-optical interrogation of the groundstate hyperfine splitting of cesium (or rubidium), compared to the optical-microwave double resonance technique conventionally employed in atomic frequency standards.
Non-patent document 5 discloses high-pressure Hall effect measurements on liquid phase epitaxial crystals of Ga1-xAlxAs with compositions in the range 0.23≤x≤0.79, which have provided information about the relative positions of the Gamma and X conduction band minima across the system.